Terahertz radiation (0.1 to 10 THz; 1 THz=1012 Hz) is situated between infrared light and microwave radiation. Terahertz (THz) science and technology is rapidly becoming a notable area of scientific research for potential applications in security, wireless communication, imaging and other areas.
As one of the most important applications in terahertz area, THz wireless communication currently subject to the attention of the world. A THz communication system has unique advantages compared to microwave and optical fiber communications. THz can support ultrahigh bandwidth spread spectrum systems, which can enable secure communication, large capacity networks, and so on. Moreover, compared to free-space infrared systems, a THz communication system has much better performance under certain atmospheric conditions (e.g., fog).
As one of the most key components of the THz communication system, a THz dynamic functional device (terahertz external modulator) has become the focus of research in the field of THz science and technology. Since terahertz band function devices require size in micron or nanometer scale, which means microwave and infrared band devices cannot be applied directly. Therefore, since 2004, Nature/Science and the top international scientific journals have published many articles about terahertz external modulators.
Researchers have tried many ways to realize the THz modulator, such as combining the Si, GaAs, phase-transition, Graphene material systems with the metamaterial. By applying the external laser, voltage and temperature changes to induce the electromagnetic characteristic change of the resonance of modulator to dynamic control the THz wave. However, until now there has been a lack of effective THz modulators which achieve fast and efficient modulation.
In recent years, with the development of semiconductor materials and technology, High Electron Mobility Transistor (HEMT) have shown excellent performance, and have been successfully applied to detectors, amplifiers, and other areas. The development of the HEMT provides new ways for the rapid response dynamic terahertz device.
HEMT is a device which applies the two dimensional electrons gas (2-DEG) to construct the transistor and has a resonance response to electromagnetic radiation at the plasma oscillation frequencies. This response can be used for new types of detectors, mixers, and multipliers. These devices should operate at much higher frequencies than conventional, transit-time limited devices, since the plasma waves propagate much faster than electrons. In 1978, R. Dingle firstly observed high electron mobility in the doped GaAs/AlGaAs super-lattices which are produced by molecular beam epitaxy (MBE). In 1980, Fujitsu developed a HEMT and successfully used it in low noise amplifier. As the third generation of wide bandgap semiconductor material GaN has not only wide band gap, but also has a large thermal conductivity, high electron saturation rate, strong breakdown field and good thermal stability, etc. Therefore, in the preparation of the high-speed dynamic devices, HEMT GaN-based material has great advantages.
Metamaterial is a kind of artificial electromagnetic array structure, which is made from assemblies of specific geometry resonance units with periodic or aperiodic patterns. The artificial designed structures gives them their smart properties capable of manipulating electromagnetic waves to achieve benefits that go beyond what is possible with conventional materials. With the development of modern micro-fabrication techniques, metamaterials played a huge role in the development of passive functional devices, and have developed a variety of related functions devices in millimeter wave, THz, and optical band.